ARM: make highpte an expert option

When highmem is enabled, there's little reason not to also enable
highpte support as well.  Rather than leaving this to chance, make
it an expert option, and default it to be enabled if highmem is
enabled.  Add some help text to the option while we're here.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>